A screen transfer technique is known in which a server apparatus carries out complicated arithmetic processing to transfer image data representing a display screen of the server apparatus. According to the screen transfer technique, a client apparatus can be implemented by a display terminal with a simple I/O interface. In the screen transfer technique, typically, image data representing the entire screen is first transferred, but subsequently, instead of the image data for the entire screen, image data representing an updated area is sequentially transferred. That is, the server apparatus needs to draw image data in accordance with operation information from a display terminal, detect an updated area, compress image data representing the updated area, and transfer the compressed image data to the display terminal.
The screen transfer technique can be applied to an image relay system. In the image relay system, an image relay apparatus relays image data from at least one relay source apparatus to at least one relay destination apparatus. That is, the image relay apparatus receives compressed image data from the relay source apparatus, decompresses the compressed image data, draws the image data, detects an updated area, makes a required change (for example, a clipping process) to the image data representing the updated area, recompresses the processed image data, and transfers the recompressed image data to the relay destination apparatus.
However, the compressed image data may not need to be changed. In such a case, the recompressed image data transferred to the relay destination apparatus matches the compressed image data received from the relay source apparatus. Thus, the series of processes including the decompression, drawing, and recompression in the image relay apparatus are useless.